[Heathkit] parasitic suppressor

[jeremy-ca]

Completely wrong information.

A parasitic is always near the upper VHF frequency limit of the tube and 
self oscillation is at or near the HF frequency the amplifier is tuned to.

Neutralization is required when interelectrode capacitances and the tank 
circuit are of the correct value to cause self oscillation at or near the 
operating frequency. A small portion of the signal is fed back to the input 
180* out of phase.

Many without sufficient knowledge of amplifier theory often confuse or 
combine the two very different items.

 A good amplifier tube makes a great oscillator. In the 20's and 30's tubes 
were run that way deliberately and were called TNT or TPTG circuits.This was 
before crystal control was developed. You can still hear these rigs on the 
air during special events, 20's or 30's night, etc; they can really sound 
great. Im still looking for a pair of 211's to build my own. The regenative 
detector as used in receivers is a form of TPTG.

Grounded grid amplifiers usually have sufficiently low gain that 
neutralization is not required but certain tubes still require attention to 
detail and short lead lengths. The 811A and 572B (same design going back to 
the 211 of 1928 and the original 811 of 1938) are marginally stable in some 
amps. The SB-200 is borderline at 10M and the FL-2100 series is less stable 
and very prone to oscillation due to the incorrect neutralizing scheme 
(feedback is in phase!!) used.

Modern amps such as the MFJ-Ameritron 811 and 572 series are neutralized.

Most ceramic/metal tubes do not require suppressors or neutralizing; the 
8877 is an excellent example of an extremely stable tube.

High gain grid driven tubes as used in exciters almost always require 
neutralization. This includes all sweep tube and 6146 rigs.

Carl
KM1H



----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <heathkit at mailman.qth.net>
Cc: <jhoyos at hoth.gcn.ou.edu>
Sent: Saturday, August 25, 2007 10:55 PM
Subject: Re: [Heathkit] parasitic suppressor


> Jorge,
>
> The C is the interelectrode capacitance *OF* the PA. What you are 
> *suppressing* are UNDESIRED harmonics, and PREVENTING or SHUNTING to 
> ground unwanted oscillation, i.e., VHF oscillation that develops in PA 
> circuits that do NOT have parasitic suppression.
>
> This is also why tube type finals in a radio have neutralizing capacitors 
> in the circuit. These are set usually at the highest band of the radio, 
> with no *real* drive. You are balancing out the inherent circuit 
> tendencies.
>
> Without blockage of parasitic oscillation, the PA very well *CAN* go into 
> a self oscillation mode and self destruct.
>
> Bob - N0DGN
>>
>> A parasitic suppressor, as *most* of us are normally involved with, 
>> creates a trap, (an L/C circuit piece), or block if you wish, that simply 
>> stops the "undesired" frequencies from passing through the finals, or the 
>> linear amplifier.
>>
>> It can be accomplished by an inductor, (coil), wrapped around a carbon 
>> resistor in the plate lead.
>>
>> Other suppression methods are in the construction phase of an 
>> amplification stage, for example:  Grounding the cathode in an amplifier 
>> tube such as 572Bs or 813s.  Bypassing 'other' grids such as control grid 
>> and screen grids with a high voltage disk ceramic capacitor from pin to 
>> ground with ALMOST *NO* lead length.
>>
>> Bob - N0DGN
>>
>>
>>
>>
>>
>>
>>
>>
>> If such a supprssor were an LC parallel circuit, its impedance were 
>> maximun
>> at the resonance frequency, so this frecuency would not be attenuated by
>> the PA because the LC circuit is in series with the anode.
>>
>> I can not figure how the attenution is accomplished.
>>
>> Tanks again.
>>
>> Jorge
>> HK6BCC
>>
>>
>>
>> --------------------------------------------------------------------
>> mail2web LIVE – Free email based on Microsoft® Exchange technology -
>> http://link.mail2web.com/LIVE
>>
>>
>> _______________________________________________
>> List Administrator: Duane Fischer, W8DBF        ** For Assistance: 
>> dfischer at usol.com **         $$ See the vintage area on the HCI web 
>> site - http://www.w9wze.org $$ Heathkit mailing list
>> Heathkit at mailman.qth.net
>> http://mailman.qth.net/mailman/listinfo/heathkit
>>
>>
>
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF        ** For Assistance: 
> dfischer at usol.com **         $$ See the vintage area on the HCI web site - 
> http://www.w9wze.org $$ Heathkit mailing list
> Heathkit at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/heathkit